Single-pole component manufacturing

ABSTRACT

The invention relates to a vertical-type single-pole component, comprising regions with a first type of conductivity which are embedded in a thick layer with a second type of conductivity. Said regions are distributed over at least one same horizontal level and are independent of each other. The regions also underlie an insulating material

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/168,040,entitled “SINGLE-POLE COMPONENT MANUFACTURING,” filed on Sep. 16, 2002,which prior application is herein incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacturing of single-polecomponents in vertical monolithic form. The following description morespecifically relates to components of Schottky diode type made invertical form in silicon substrates.

2. Discussion of the Related Art

FIG. 1 illustrates a conventional Schottky diode structure. Such astructure includes a semiconductor substrate 1, typically made ofheavily-doped single-crystal silicon of a first conductivity type,generally N type. A cathode layer 2 covers substrate 1. It is N-typedoped, but more lightly than substrate 1. A metal layer 3 forms aSchottky contact with N-type cathode 2.

The thickness of layer 2 is chosen to determine the reverse breakdownvoltage of the Schottky diode.

FIG. 2 illustrates the variation of the electric field E across thethickness of the structure shown in FIG. 1, along an axis A-A′. Forclarity, the different portions of curve 10 of FIG. 2 have beenconnected by dotted lines to the corresponding regions of FIG. 1.

In such a homogeneous structure, the field variation per thickness unitis proportional to the doping level. In other words, the field decreasesall the faster as the doping is heavy. It thus very rapidly drops to azero value in substrate 1. Since the breakdown voltage corresponds tothe surface included between the axes and curve 10, to obtain a highbreakdown voltage, the doping of layer 2 must be minimized and itsthickness must be maximized.

In the manufacturing of single-pole components, opposite constraintshave to be considered. Single-pole components, such as the diode shownin FIG. 1, must indeed have as small a resistance (Ron) as possible,while having as high a breakdown voltage as possible when reversebiased. Minimizing the on-state resistance of a single-pole componentimposes minimizing the thickness of the most lightly doped layer (layer2) and maximizing the doping of this layer.

To optimize the breakdown voltage without modifying resistance Ron,structures of the type of that in FIG. 3 have been provided. In FIG. 3,a vertical Schottky diode includes a single-crystal siliconsemiconductor substrate 31, heavily doped of a first conductivity type,for example, type N, and coated with a layer 32. Layer 32 is formed ofthe same semiconductor material as substrate 31 and is of same dopingtype, but more lightly doped. Layer 32 is intended for forming thecathode of the Schottky diode. A metal layer 33 covers layer 32. Themetal forming layer 33 is chosen to form a Schottky contact with N-typesilicon 32.

Layer 32 includes very heavily-doped P-type silicon regions or “islands”34. Islands 34 are distributed over at least one horizontal level (overtwo levels in the example of FIG. 3).

Islands 34 are separate and buried in layer 32. The islands 34 ofdifferent horizontal levels are substantially distributed on samevertical lines.

FIG. 4 illustrates the variation profile of electric field E across thethickness of a structure similar to that in FIG. 3. More specifically,the profile of FIG. 4 is observed along axis A-A′ of FIG. 3.

As appears from the comparison of FIGS. 2 and 4, the insertion ofheavily-doped P-type “islands” 34 in the structure of FIG. 3 modifiesthe variation of field E per thickness unit. Since islands 34 are muchmore heavily doped than N-type layer 32, there are more negative chargescreated in islands 34 than there are positive charges in layer 2. Thefield thus increases back in each of the horizontal areas includingislands 34. By setting the doping and the number of islands 34, thespace charge area can be almost indefinitely widened. In reversebiasing, the cathode formed by layer 32 and islands 34 thus generallybehaves as a quasi-intrinsic layer. In average, the electric fieldvariation per thickness unit thus strongly decreases. Thus, for a givendoping level of layer 32, the breakdown voltage is increased, asillustrated by the increase of the surface delimited by the axes and thecurve of FIG. 4 as compared to the corresponding surface of FIG. 2.

Accordingly, the structure of FIG. 3 enables obtaining single-polecomponents of given breakdown voltage with a resistance Ron smaller thanthat of a conventional structure.

The practical implementation of such a structure with islands isdescribed, for example, in German patent 19,815,907 issued on May 27,1999, in patent applications DE 19,631,872 and WO99/26,296, and inFrench patent 2,361,750 issued on Mar. 10, 1978. These differentdocuments provide obtaining a structure similar to that in FIG. 3 byperforming implantations/diffusions during a growth epitaxy of layer 32.

The repeated interruptions of the epitaxial growth are a disadvantage ofsuch an implementation. Indeed, thick layer 32 thus obtained has anirregular structure. Such structure irregularities alter theperformances of the final component.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel method formanufacturing single-pole components of vertical type having adetermined breakdown voltage and having a reduced on-state resistance.The present invention also aims at the obtained components.

To achieve these objects, the present invention provides a single-polecomponent of vertical type, including regions of a first conductivitytype buried in a thick layer of a second conductivity type, said regionsbeing distributed over at least one same horizontal level and beingindependent from one another, and the independent regions of whichunderlie an insulating material.

According to an embodiment of the present invention, the componentincludes at least two levels, the independent regions of successivelevels being substantially vertically aligned.

According to an embodiment of the present invention, the independentregions are rings.

According to an embodiment of the present invention, the deepest levelincludes non ring-shaped regions.

The present invention also provides a method for manufacturing asingle-pole component of vertical type in a silicon substrate of a givenconductivity type, including the steps of:

-   -   a) forming openings in a thick silicon layer covering the        substrate, doped of the same conductivity type as said        substrate, but more lightly;    -   b) coating the walls and bottoms of the openings with a silicon        oxide layer;    -   c) forming, by implantation/diffusion through the opening        bottoms, regions of the conductivity type opposite to that of        the substrate; and    -   d) filling the openings with an insulating material.

According to an embodiment of the present invention, before step d) offilling the openings, steps a) to c) are repeated at least once, theinitial openings being continued into the thick silicon layer.

According to an embodiment of the present invention, the silicon layerof the same given type of conductivity as the substrate is intended forforming the cathode of a Schottky diode.

According to an embodiment of the present invention, the silicon layerof same conductivity type as the substrate is intended for forming thedrain of a MOS transistor.

The foregoing and other objects, features and advantages of the presentinvention, will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates, in a partial simplified cross-section view, aconventional Schottky diode structure;

FIG. 2 illustrates the variation of the electric field across thethickness of the structure of FIG. 1;

FIG. 3 illustrates, in a partial simplified cross-section view, thestructure of a Schottky diode having a determined breakdown voltage anda reduced on-state resistance;

FIG. 4 illustrates the variation of the electric field across thethickness of the structure of FIG. 3; and

FIGS. 5A to 5D are partial simplified cross-section views of a Schottkydiode at different steps of a manufacturing process according to thepresent invention.

For clarity, the same elements have been designated with the samereferences in the different drawings. Further, as usual in therepresentation of integrated circuits, the drawings are not to scale.

FIGS. 5A to 5D illustrate steps of a manufacturing method of a verticalmonolithic Schottky diode according to the present invention.

DETAILED DESCRIPTION

As illustrated in FIG. 5A, a substrate 61 is initially covered with asingle-crystal silicon layer 62, of same doping type, for example N, assubstrate 61. Layer 62, intended for forming the cathode of the Schottkydiode, is more lightly doped than substrate 61. Layer 62 is etched, bymeans of a mask 65, to form openings 66. Substrate 61 and layer 62 areobtained by any appropriate method. For example, layer 62 may resultfrom an epitaxial growth on substrate 61, or substrate 61 and layer 62may initially be a same semiconductor region, the doping differencesthen resulting from implantation-diffusion operations.

At the next steps, illustrated in FIG. 5B, an insulating layer 67, forexample a silicon oxide layer (SiO₂), is formed on the walls and at thebottom of openings 66. Then, a P-type dopant that penetrates into thesilicon at the bottom of openings 66 is implanted, after which a heatingis performed to form heavily-doped P-type regions 641.

At the next steps, illustrated in FIG. 5C, layer 67, regions 641, andlayer 62 are anisotropically etched, to form openings 68 that continueopenings 66. The upper portion of each of openings 68 is thus surroundedwith a diffused ring 641. Then, the walls and bottoms of openings 68 arecovered with a thin insulating layer 69, for example silicon oxide.

The implantation operations previously described in relation with FIG.5B are then repeated to form heavily-doped P-type regions 642.

At the next steps, illustrated in FIG. 5D, openings 66-68 are filledwith an insulating material 70. Then, mask 65 is removed and thestructure thus obtained is planarized. Finally, a metal layer 63 adaptedto ensuring a Schottky contact with layer 62 is deposited over theentire structure.

Before ending, in accordance with the steps described in relation withFIG. 5D, the structure formation by removing mask 65, filling openings66-68 with material 70, and depositing a metal layer 63, the stepsdescribed in relation with FIG. 5C could be repeated several times, toform several horizontal levels of heavily-doped P-type rings similar torings 641.

It should be noted that the intermediary rings and the underlyingregions form islands according to the preceding definition. They thusprovide the corresponding advantages, previously discussed in relationwith FIGS. 3 and 4.

An advantage of the method according to the present invention and of theresulting structure, previously described in relation with FIG. 5D, isthe forming of a homogeneous cathode region 62.

Those skilled in the art will know how to adapt the number, thedimensions, the positions, and the doping of the different rings 641,642 to the desired performances. As an example, according to prior art,to obtain a breakdown voltage of approximately 600 volts, a cathodelayer (2, FIG. 1) of a thickness of approximately 40 μm and of a dopinglevel on the order of 2.2·10¹⁴ atoms/cm³ may be used, which results inan on-state resistance of approximately 6.7 Ω·mm². According to thepresent invention, by using groups of three P-type rings doped atapproximately 3.5·10¹⁷ atoms/cm³, vertically spaced apart by 10 μmaround silicon oxide columns of a 1-μm width, for a same breakdownvoltage of 600 V with an epitaxied layer (62, FIG. 5D) of a samethickness on the order of 40 μm, the cathode doping could be increasedto a value on the order of some 10¹⁵ atoms/cm³, which results in anon-state resistance of approximately 3 Ω·mm².

It should be noted that it has been chosen to describe as a non-limitingexample the present invention in relation with FIG. 6 applied to theforming of silicon islands in the cathode of a Schottky diode. It wouldhowever be possible to implement a method aiming at forming in the drainof a MOS transistor, around vertical columns of an insulating material,very heavily-doped P-type silicon rings, similarly to the methodpreviously described in relation with FIGS. 5A-D.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the operations described in relationwith FIG. 5D can be carried out according to any appropriate sequence.Thus, after filling openings 66-68, layer 65 may be removed and thestructure may be planarized in a single step by means of a chem-mechpolishing (CMP) method.

Further, the present invention applies to the forming in vertical formof any type of single-pole component, be it to reduce its on-stateresistance for a given breakdown voltage, or to improve its breakdownvoltage without increasing its on-state resistance.

1. A method of manufacturing a single-pole component of vertical type ina silicon substrate of a first conductivity type, comprising the stepsof: a) forming openings in a thick silicon layer of the firstconductivity type covering the substrate, the thick silicon layer beingmore lightly doped than the substrate; b) coating walls and bottoms ofthe openings with a silicon oxide layer; c) forming, byimplantation/diffusion through the bottoms of the openings, regions of asecond conductivity type opposite to that of the substrate; and d)filling the openings with an insulating material.
 2. The method of claim1, wherein, before step d), steps a), b) and c) are repeated at leastonce, with the openings being continued into the thick silicon layer. 3.The method of claim 2, wherein the thick silicon layer forms a cathodeof a Schottky diode.
 4. The method of claim 2, wherein the thick siliconlayer forms a drain of a MOS transistor.
 5. The method of claim 1,wherein the thick silicon layer forms a cathode of a Schottky diode. 6.The method of claim 1, wherein the thick silicon layer forms a drain ofa MOS transistor.